PageRank (PR) is used by the web search engine Google in ranking individual web pages. This method is described in U.S. Pat. No. 6,799,176. To calculate PR for a web page, all of its inbound links are taken into account, including links from within the site and links from outside the site. When calculating PR, pages with no outbound links are assumed to have linked out to all other pages in the collection. Their PR scores are therefore divided evenly among all other pages. PR of a web page A is calculated by the equation:
      PR    ⁡          (              p        i            )        =                    (                  1          -          d                )            N        +          d      ⁡              (                                            PR              ⁡                              (                                  t                  1                                )                                                    C              ⁡                              (                                  t                  1                                )                                              +                                    PR              ⁡                              (                                  t                  2                                )                                                    C              ⁡                              (                                  t                  2                                )                                              +          …          +                                    PR              ⁡                              (                                  t                  n                                )                                                    C              ⁡                              (                                  t                  n                                )                                                    )            where t1 to tn are pages linked to page A, C is the number of outbound links of a particular page, and d is a damping factor, usually being set to 0.85 to reduce influence of pages acting ask “sinks”.
While the PR is considered to be accurate at the time of publication by Google for most sites, it is known that this value is also easily manipulated. For example, any low page with a low PR value that is redirected, for instance to a “Refresh” meta tag, to a high PR page causes the lower PR page to acquire the PR of the destination page. In theory a new PR 0 page with no incoming links can be redirected to the Google home page, which is a PR 10 page. By the next update the PR of the new page will be upgraded to 10. This is known as spoofing and is a known failing or bug in the calculation of PR. Any page's PR can be manipulated or “spoofed” to a higher or lower value.
Further, the calculation of PR will require iterative cycles of computations to achieve a “steady” value. This would mean that huge computation resources are required to obtain reasonably reliable PR values for various web pages.